Method for melting titanium base alloys

ABSTRACT

THE PRESENT INVENTION RELATES TO A METHOD FOR PRODUCING ALLOYS CONTAINING EFFECTIVE AND SUBSTANTIAL QUANTITIES OF MOLYBDENUM. MORE PARTICULARLY, THE INVENTION RELATES TO A METHOD FOR PRODUCING MOLYBDENUM-CONTAINING, TITANIUM-BASE ALLOYS EXHIBITING A SUBSTANTIALLY HOMOGENEOUS MICROSTRUCTURE WHEREIN THE MOLYBDNUM IS DISSOLVED AND DISPERSED SUBSTANTIALLY UNIFORMLY THROUGHOUT THE MATRIX METAL. THIS IS ACHIEVED BY PROVIDING A CHARGE OF POROUS MOLYBDENUM PARTICLES, MIXING SAID CHARGE OF POROUS MOLYBDENUM PARTICLES, MIXING SAID CHARGE WITH A CHARGE OF PARTICULATE MATERIAL, WHICH MAY BE TITANIUM, ZIRCONIUM OR BASE ALLOYS THEREOF, FORMING A CONSUMABLE ELECTRODE FROM SAID MIXTURE, AND VACUUM ARC MELTING SAID CONSUMABLE ELECTRODE TO FORM THE DESIRED HOMOGENEOUS ALLOY.

Jan. 5, 1971 v c PETERSEN ET AL 3,552,947

METHOD FOR MELTING TITANIUM-BASE ALLOY Filed Jan. 18, 1968 2Sheets-Sheet 1 INVENTORS. VINCENT C. PETERSEN 8 RUSSELL C.BUEHL A/forneyJan. 5, 1971 v, Q PETERSEN ET AL 3,552,947

METHOD FOR MELTING TITANIUM-BASE ALLOY Filed Jan. 18, 1968 2Sheets-Sheet fl INVENTORS. VINCENT C. PETERSEN 8 RUSSELL C. BUEHL A fforney United States Patent 3,552,947 METHOD FOR MELTING TITANIUM- BASEALLOYS Vincent C. Petersen, North Fayette Township, Allegheny County,and Russell C. Buehl, Beaver, Pa., assignors to Crucible Inc., acorporation of Delaware Filed Jan. 18, 1968, Ser. No. 698,795 Int. Cl.C2211 7/00; C22c 15/00 US. CI. 75-10 Claims ABSTRACT OF THE DISCLOSUREThe present invention relates to a method for producing alloyscontaining effective and substantial quantities of molybdenum. Moreparticularly, the invention relates to a method for producingmolybdenum-containing, titanium-base alloys exhibiting a substantiallyhomogeneous microstructure wherein the molybdenum is dissolved anddispersed substantially uniformly throughout the matrix metal. This isachieved by providing a charge of porous molybdenum particles, mixingsaid charge with a charge of particulate material, which may betitanium, zirconium or base alloys thereof, forming a consumableelectrode from said mixture, and vacuum arc melting said consumableelectrode to form the desired homogeneous alloy.

In the production of molybdenum-containing alloys, difiiculties areencountered in achieving complete dissolution of the highly refractorymolybdenum when alloyed with metals having melting points lower thanmolybdenum, particularly metals such as titanium and zirconium.Molybdenum has a melting point of 2610 0., whereas, zirconium has amelting point of 1852 C., and titanium has a melting point of 1668 C.Molybdenum-containing alloys of this type are conventionally produced byforming a consumable electrode of the base metal and alloying elementsby compacting particles thereof. The consumable electrode is are meltedin a vacuum or inert atmosphere to form an ingot. Thereafter, the ingotis used to produce additional electrodes, which are again vacuum arcmelted. This procedure is repeated until the desired purity anddissolution of the alloying elements are achieved. Although, because ofits highly refractory nature, molybdenum is difficult to dissolve,nevertheless, this problem can be overcome by numerousconsumable-electrode meltings. Specifically, as many as ten electrodemeltings may be required for this purpose. This obviously renders theoperation so expensive as not to be commercially feasible. Moreover,these numerous meltings introduce productquality problems, such asexcessively high oxygen content. In addition, since molybdenum is of asubstantially greater density than the base alloy titanium, inconsumable-electrode melting operations the molybdenum particles tend,during the electrode-melting operation, to drop in unmelted form fromthe electrode into the molten pool of the ingot and segregate to thebottom thereof before dissolution. This results in a segregation of themolybdenum in the resulting solidified ingot, even if the molybdenum issubjected to sufficient time at temperature to dissolve the metal. Thissegregation carries over through any remelts so that the final productis unhomogeneous. It may be seen, therefore, that this problem cannot beovercome by numerous consumable-electrode meltings, as is the case withthe problem of achieving dissolution of the molybdenum particles. Theproblem with respect to segregation of the molybdenum during melting maybe more fully appreciated when it is noted that the specific gravity ofmolybdenum is more than twice that of titanium; molybdenum has aspecific gravity of 10.2, whereas, titanium has a specific gravity of4.5.

3,552,947 Patented Jan. 5, 1971 It is, therefore, a primary object ofthe present invention to provide a method for producing homogeneousalloys of molybdenum and a lower melting point, lower density basemetal, such as titanium or zirconium.

This, as well as other objects of the invention, as well as a completeunderstanding thereof, will be apparent from the following descriptionand drawings, in which:

FIG. 1 is a photomicrograph of a molybdenum-containing titanium-basealloy produced in the conventional manner and exhibiting acharacteristic dense, unmelted molybdenum inclusion;

FIG. 2 is a photomicrograph of a molybdenum-containing titanium-basealloy, substantially the same as the alloy of FIG. 1, produced inaccordance with the method of the present invention and exhibiting ahomogeneous microstructure free from segregated dense molybdenuminclus10ns;

FIG. 3 is a radiograph of the alloy of FIG. 1 showing the presence ofdense, unmelted molybdenum inclusions in the alloy resulting frommolybdenum segregation;

FIG. 4 is a radiograph of the alloy of FIG. 2 showing the desiredhomogeneous microstructure.

The problem of unhomogeneous molybdenum-containing alloys resulting frommolybdenum segregation during melting is avoided in the practice of thepresent invention by using molybdenum particles of relatively lowdensity in admixture with the base metal in producing the consumableelectrode. For this purpose it has been found that any decrease in thedensity of the molybdenum particles will have a beneficial effect withrespect to the homogeneity of the resulting ingot produced during vacuumarc melting. Preferably, it is desired that the particles have aspecific gravity of less than 7.4 and for optimum results within therange of 5 to 7.4. By using molybdenum particles having a specificgravity within this range, substantially homogeneous alloys may beobtained through conventional vacuum arc melting techniques. It isunderstood, of course, that in the conventional manner multiple meltingsare employed to enhance the purity of the alloy and insure substantiallycomplete dissolution of the alloying elements, particularly the highmelting point molybdenum.

In its broadest aspects, therefore, the method of the inventioncomprises steps of producing a charge of molybdenum particles having aspecific gravity less than about 10 and preferably within the range of 5to 7.4, admixing said particles with a charge of a particulate materialselected from the group consisting of titanium, zirconium and basealloys thereof, forming said admixture into a consumable electrode andvacuum arc melting said electrode in a vacuum or inert atmosphere.

The low density molybdenum particles consist of porous molybdenumagglomerates. The agglomerates are preferably of substantially the samesize consist as the particles of the other elements making up theelectrode compact. Typically, this is about minus 8 mesh (US. StandardSieve). By maintaining the particles of all the elements ofsubstantially the same size separating-out the particles, particularlytitanium and molybdenum particles, upon agitation of the admixtureduring transport incidental to consumable-electrode manufacture issubstantially avoided. This is necessary if segregation of the alloyingelements, particularly molybdenum, is to be avoided during thesubsequent manufacture and vacuum arc melting of the electrode. If thealloying elements are segregated in the electrode, they will also besegregated in the resulting ingot melted therefrom.

Although it is possible to produce the desired low density molybdenumparticles by various techniques, it is presently preferred to producethe agglomerates by the following technique.

Molybdenum is typically produced by hydrogen reduction of molybdenumoxide at elevated temperatures.

At suitable temperatures, for example 1200 to 1400 C., the basicallymicron-sized molybdenum powder so produced will sinter into low-densityagglomerates. Such agglomerates are satisfactory for the practice ofthis invention.

As a specific example of the practice of the invention, the followingmolybdenum-containing titanium-base al- 10y was melted both in theconventional manner and in accordance with the present invention:

Percent Molybdenum 11.5 Zirconium 6 Tin 4.5 Titanium Balance The abovealloy, produced in accordance with the present invention, used minus 6mesh particles of the alloy constituents with molybdenum agglomerateshaving a specific gravity of about 6.4 and produced in accordance withthe technique outlined hereinabove. The alloying elements in particleform were pressed into bricks. The bricks were then welded in an inertatmosphere to form a consumable electrode, which was then melted in anevacuated furnace to form an ingot. Such operation was performed inaccordance with well known vacuum arc melting techniques.

A similar consumable electrode of the above alloy was prepared in theconventional manner using dense molybdenum turnings having a specificgravity of 10.2 instead of low density agglomerates in accordance withthe invention. This alloy was melted in the same manner as thatdescribed above in accordance with the present invention. The onlydifference between the two operations was that in one dense molybdenumturnings were used, in accordance with conventional practice, and in theother low density molybdenum agglomerates were used in place of themolybdenum turnings, in accordance with the present invention. After twomelting operations, photomicrographs of the ingot produced in accordancewith the conventional techniques using high density molybdenum turningand that produced in accordance with the present invention were taken;the former is identified as FIG. 1 and the latter as FIG. 2 of thedrawings. In the center of FIG. 1 there is shown a typical densemolybdenum inclusion resulting from the failure of the molybdenum todissolve in the molten metal caused by its segregation upon droppingfrom the electrode and into the molten pool of the ingot. Because of itshigh density, the molybdenum particle quickly sank within the moltenpool of titanium and reached the bottom thereof without completelydissolving. Because of the relatively lower temperature prevailing atthe bottom of the ingot, which in the case of a titanium-base alloy isnear the solidus temperature of titanium, the molybdenum particle didnot dissolve but remained as an unmelted inclusion in the microstructureof the alloy. In contrast, the photomicrograph of FIG. 2, which is of analloy produced in accordance with the invention, shows the completeabsence of undissolved molybdenum inclusions. This results from the factthat the molybdenum particles as they dropped from the electrode end andinto the molten pool were of a relatively low density. Consequently,they did not sink to the bottom of the molten titanium prior todissolving. FIG. 3, which is a radiograph of the ingot of FIG. 1, againshows, as black areas, the presence of segregated undissolvedmolybdenum. In contrast, FIG. 4, which is a radiograph of the ingot ofFIG. 2, shows the complete absence of segregated undissolved molybdenum.I

Although various embodiments of the invention hav been shown anddescribed herein, it is obvious that other adaptations and modificationsmay be made by those skilled in the art without departing from the scopeand spirit of the appended claims.

What is claimed is:

1. A method of producing substantially homogeneous alloys containingeffective quantities of molybdenum, comprising producing a charge ofmolybdenum particles, said particles having a specific gravity of lessthan 7.4, admixing said molybdenum particles with a charge of particlesof at least one metal, which includes at least one metal selected fromthe group consisting of titanium, zirconium and base alloys thereof,forming said admixture into a consumable electrode, and vacuumarcmelting said electrode, whereby the density of said molybdenumparticles delays the sinking thereof in the resulting melt of lesserspecific gravity.

2. A method according to claim 1 wherein said molybdenum chargeconstitutes porous molybdenum agglomerates.

3. A method according to claim 2 wherein said molybdenum agglomeratesare about minus 6 mesh.

4. A method according to claim 2 wherein said mo lybdenum agglomeratesand said charge are particles of substantially the same size, wherebysegregation of the materials is minimized.

5. A method according to claim 2 wherein said molybdenum agglomerateshave a specific gravity of about 5 to 7.4.

References Cited UNITED STATES PATENTS 2,550,735 5/1951 Tour 75-532,702,239 2/1955 Gilbert 75-10 2,848,315 8/1958 Kieffer 75-10 3,005,24610/1961 Murphy 75-10 3,101,267 8/1963 Dunn 75-10 3,269,825 8/1966Vordahl 75-10 FOREIGN PATENTS 838,586 6/1960 United Kingdom 75-10WINSTON R. DOUGLAS, Primary Examiner P. D. ROSENBERG, Assistant ExaminerUS. Cl. X.R. 75115.5

